1. Field of the Invention
The present invention relates to a vehicular active vibratory noise control apparatus for canceling road noise generated in the passenger compartment of a vehicle when the vehicle is driven, by causing a canceling sound, which is in opposite phase with and equal in amplitude to the road noise, to interfere with the road noise.
2. Description of the Related Art
Heretofore, there has been proposed in the art a vehicular active vibratory noise control apparatus for canceling road noise (also called “drumming noise”) in the passenger compartment of a vehicle, with a canceling sound that is in opposite phase with the road noise at an evaluation point (hearing point) where a microphone is located (see Japanese Laid-Open Patent Publication No. 2007-025527). The road noise is based on vibrations of vehicle wheels, which are caused by the road when the vehicle is running thereon. Such vibrations are transferred through the suspension to the vehicle body, and are excited by the acoustic resonant characteristics of the closed passenger compartment. The road noise has a peak level at a frequency of about 40 [Hz], and has a frequency bandwidth in a range from 20 to 150 [Hz].
In addition to the first peak referred to above at about 40 Hz, the road noise also has a second peak at a frequency of about 70 Hz.
FIG. 10 of the accompanying drawings shows in block form a vehicular active vibratory noise control apparatus 200 for canceling road noise at two frequencies of 40 Hz (ω0=2π×40) and 70 Hz (ω1=2π×70), based on the technique disclosed in Japanese Laid-Open Patent Publication No. 2007-025527.
The vehicular active vibratory noise control apparatus 200 has two processing circuits 201A, 201B including respective sine-wave generating means 202 for generating sine waves having respective frequencies of 40 Hz and 70 Hz, respective cosine-wave generating means 203 for generating cosine waves having respective frequencies of 40 Hz and 70 Hz, respective pairs of one-tap digital filters 204, 205 for processing the output signals from the sine-wave generating means 202 and the cosine-wave generating means 203, and respective pairs of coefficient updating means 206, 207 for sequentially updating coefficients of the corresponding one-tap digital filters 204, 205. Each of the processing circuits 201A, 201B supplies output signals therefrom to an adder 211, which adds the output signals into a signal whose amplitude and phase are adjusted by an adjusting circuit 208. The adjusted signal is supplied from the adjusting circuit 208 to a speaker 209, which radiates a canceling sound. A microphone 210 detects an interference sound generated by interference between the canceling sound and the road noise, and inputs an output signal, which is representative of the detected interference sound, to the processing circuits 201A, 201B.
FIG. 11A of the accompanying drawings shows a characteristic curve 212 of the road noise input to the processing circuit 201A, together with a characteristic curve 214 of the output signal from the processing circuit 201A. FIG. 11B of the accompanying drawings shows a characteristic curve 212 of the road noise input to the processing circuit 201B, together with a characteristic curve 214 of the output signal from the processing circuit 201B. As shown in FIG. 11A, the characteristic curve 214 is of the same amplitude as the characteristic curve 212 at 40 Hz, but is lower in amplitude than the characteristic curve 212 at 70 Hz. Conversely, as shown in FIG. 11B, the characteristic curve 214 is of the same amplitude as the characteristic curve 212 at 70 Hz, but is lower in amplitude than the characteristic curve 212 at 40 Hz. Therefore, the processing circuits 201A, 201B affect each other in operation at all times, which tends to cause the processing circuits 201A, 201B to become unstable in operation.
In addition, there are certain technical difficulties that occur when a single adjusting circuit 208 is utilized to adjust road noise components of 40 Hz and 70 Hz in amplitude and phase.